Duct system fires pose real challenges

A fire in ventilation ductwork last week resulted in the evacuation of an automotive manufacturing plant in Michigan. Crews spent nearly two hours isolating the fire within some very large ducts used in the manufacturing process.

Ventilation ducts are used in many facilities, whether for dust extraction, material handling, or air quality management. In every case, fires within ductwork present several concerns for firefighters. Here are just a few…

Ductwork circulates fire just as well as it does air, so flames and smoldering materials can rapidly spread throughout a plant’s duct system, quickly involving a large area within a plant. In this case, the article mentions ducts being involved over an area of 20,000 square feet. While fire codes dictate the placement of duct detectors in certain installations of HVAC equipment, the same precautions are often purely voluntary in industrial duct applications. I suggest that rate-of-rise heat detectors be installed within ducts or filter units so that a sudden increase in temperature will perform a shut-down of blower fans, thus limiting the spread and continued air flow in a fire situation.

Ventilation systems in manufacturing plants are typically installed to move particulate matter within an air stream, often through a filtration unit. If this particulate matter meets the criteria of a combustible dust, you know have a potential for an explosion within the duct system. Beyond the obvious hazard to firefighters, an explosion may transmit fire in directions opposite of the typical material flow, now sending fire and burning material into additional areas within a plant that were not previously involved. This is where proper engineering of systems to include back-flow dampers can reduce the risk of fire and smoke traveling into unaffected areas of a plant.

The introduction of fire suppression water into a duct system (either via automatic sprinklers or via manual hose streams) can quickly create problems with overhead loading. remember, duct supports are typically engineered to support the duct and a minimal amount of accumulation within the system. Simply adding water to the interior of the ducts without a proper means of draining the system will quickly overload supports and potentially bring ductwork down on top of personnel. One duct fire I investigated in a client’s facility resulted in the fire department adding water into the roof exhausts — and then after ducts and supports began to visibly sag, fire personnel punched holes in the ducts with claw hammers to get water to drain out. Since this is not a recommended practice where combustible dust may be involved, it is preferable to include access hatches within duct systems so that firefighters can quickly access the interiors of ducts and drain water without causing more damage and potentially creating a more dangerous scenario.

Without a doubt, proper engineering of duct systems can make or break a fire response. Proper safeguarding can include a number of approaches, from isolation dampers to spark or heat detection systems to automatic sprinklers. This is an excellent example of pre-emergency planning and inspection serving you well in terms of response efficiency and firefighter safety.

This topic and others relating to pre-emergency planning and inspection of manufacturing and industrial facilities will be included in my pre-conference workshop at FDIC International in Indianapolis on April 20.

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